1. Field of the Invention
The present invention relates to a piezoelectric material having a new composition, in particular, a piezoelectric material that does not contain lead. Additionally, the present invention relates to a piezoelectric material that does not contain an alkaline metal bringing about disturbance in processing treatment when applied to the MEMS (Micro Electro Mechanical Systems) technique.
2. Description of the Related Art
Piezoelectric materials are widely used in applications such as actuators, ultrasonic vibrators, micro power supplies and high voltage generators. The piezoelectric materials used in these devices and apparatuses are perovskite structure materials referred to as PZT, and high both in Curie temperature and in piezoelectric property; however, such materials contain lead and hence exert a large environmental load. Accordingly, from the viewpoint of environmental issues, the development of non-lead piezoelectric materials that do not contain lead is being advanced.
Although as examples of the non-lead piezoelectric materials, a large number of piezoelectric materials containing alkaline metals such as KxNa(1−x)NbO3 are known, such materials use alkaline metals and hence when alkaline metal-containing piezoelectric films are formed in micromachines such as MEMSs that use Si substrates, problems are caused in the micromachine production processes.
For example, in a case where a piezoelectric film is formed on a Si substrate, when a piezoelectric material is crystallized, the elements contained in the piezoelectric material are made to diffuse onto the Si substrate, and hence the rate of the etching processing of the Si substrate is changed to cause a problem in device fabrication. Additionally, flowing an alkaline metal-containing material in a process step results in a contamination by an alkaline metal to cause a similar production process-related problem.
Such alkaline metal-containing piezoelectric materials also have a problem of tending to undergo environmental degradation.
Ba-containing piezoelectric materials are among the piezoelectric materials that do not contain any alkaline metal and are additionally of non-lead type. For example, there has hitherto been known BaTiO3 which is an ABO3 type perovskite oxide and contains an alkaline earth metal as the A-site element. Although BaTiO3 is extremely large in the piezoelectric property (electric field-induced distortion), the Curie temperature (Tc) thereof is as low as approximately 130° C., and hence is not sufficiently satisfactory as a material for devices such as piezoelectric actuators and ultrasonic vibrators required to have high Tc values.
Also likewise, BaCu1/3Nb2/3O3 has hitherto been known as a Ba-containing piezoelectric material. This piezoelectric material has a Curie temperature as extremely high as approximately 520° C., but has an electric field-induced distortion representing the magnitude of the piezoelectric property as extremely low as 0.002%, and hence is not practical as a piezoelectric material (BaCu1/3Nb2/3O3 has a c/a ratio of 1.031, and is a tetragonal crystal).
Although, as described above, piezoelectric materials large only in piezoelectric property or piezoelectric materials high only in Curie temperature have been known, materials large both in piezoelectric property and in Curie temperature have never been known except for lead-containing or alkaline metal-containing piezoelectric materials.
The development of materials, as materials similar to BaTiO3, each having a high Curie temperature while maintaining the piezoelectric property has been attempted by combining various materials, but any of such developed materials is not sufficiently satisfactory. For example, Japanese Patent Application Laid-Open No. S63-288917 describes, as a material for condensers, Ba(Zn, Ta)O3-Ba(Zn, Nb)O3 obtained by forming a solid solution of two Ba-containing piezoelectric materials which are the same in crystalline phase; however, this material is too low in piezoelectric property to be suitable for practical applications.
Additionally, an attempt has been made to include Cu in a Ba-containing perovskite structure material, this material being not a piezoelectric material. Japanese Patent Application Laid-Open No. H06-61017 describes a material composed of (Sr, Ba, Ca)(Ti, Nb, Cu)O3 as a varistor material prepared by including Cu in a Ba-containing perovskite structure material, with the Cu content thereof ranging from 0.01 atomic percent (atm. %) to 1 atm. %. However, when the material described in Japanese Patent Application Laid-Open No. H06-61017 is used as a piezoelectric material, Tc (Curie temperature) is lower than 130° C. and the piezoelectricity is lost in high-temperature environments. Additionally, when the Cu content in this material exceeds 2%, the dielectric loss of this material becomes too large to be qualified as a piezoelectric material.
As described above, piezoelectric materials in each of which both the Curie temperature and the piezoelectric property (electric field-induced distortion) are high have never hitherto been found except for lead-containing materials and alkaline metal-containing materials.
The present invention has been achieved in view of the above-described background art, and the problem to be solved by the present invention is to provide a piezoelectric material that can be applied even to the MEMS technique, exhibits satisfactory piezoelectricity even at high ambient temperatures and is environmentally clean.